MAMMAKY SECRETION. 623 



In herbivora, carnivora, and omnivora, therefore, the same general 

 rule holds, that an increase in the albuminous constituents of the food 

 increases both the total amount of milk and its percentage in fats. In 

 cows, however, it is to be noted that the relative percentages of casein 

 and fat are not dependent so much on the amount of albumen of the 

 food as on the breed and individual characteristics. The influence of the 

 fatty constituents of the food on the composition of the milk is less 

 marked. 



It will be shown, under the subject of Nutrition, that the addition 

 of fat to the food serves to reduce the waste of tissue-albumen, and 

 therefore permits the deposition of nitrogenous tissue-constituents. To 

 this extent, by yielding a greater supply of albuminous bodies to the 

 glandular epithelium, a fatty diet may help the milk secretion, but not, 

 as will be shown directly, by an immediate transfer of the fat of the food 

 to the milk. In fact, the addition of fat to the food even seems to reduce 

 the amount of butter in the milk if the amounts of albuminoids in the 

 food are not amply sufficient for the nutritive needs of the economy. 

 When, however, the albuminoids and other constituents of the food are 

 ample for preserving nutritive equilibrium, the further addition of fat 

 will increase the percentage of butter in the milk. 



The milk secreted at different hours of the day shows certain con- 

 stant, though small, variations in composition. 



Morning milk has the largest percentage of water; midday milk the 

 smallest. 



In 100 Parts. - Morning. Noon. Evening. 



So/also, the first- and last-drawn portions of milk have different com- 

 positions. The last portions have more solids, and especially more fat, 

 than the earlier portions. A difference of four hours in time of milking 

 makes this most apparent. It has been attempted to attribute this differ- 

 ence to the rising of the cream in the udder of the cow, just as occurs in 

 milk standing in a vessel outside the body. The same differences may, 

 however, be made out in human milk, where this mechanical explanation 

 can have but little force. It must not be forgotten that in milking not 

 only ready-formed milk is withdrawn, but during the act new milk is 

 secreted, and it is quite warrantable to suppose the cell processes which 

 result in the production of the solids of the milk are less active in the 

 pauses than during the act of milking or suckling, when the process is 

 stimulated from the irritation of the nipples. 



